Paper EM+NS+SS+TF-FrM3
Photoluminescence and Kelvin Probe Studies of Mg-doped GaN

Friday, November 1, 2013, 9:00 am, Room 101 B

High-quality p-type gallium nitride (GaN) can only be achieved by doping with magnesium (Mg). However, some unidentified point defects are also present in this material and detrimentally affect the properties of GaN thin films and related devices. Analyzing the photoluminescence (PL) spectra of GaN thin films provides valuable information regarding point defects in GaN. Additionally, the Kelvin probe method supplies complementary information about the electrical and optical properties of GaN near the surface. We have demonstrated in the past that simple phenomenological models based on rate equations are able to describe the results obtained in PL and Kelvin probe experiments. In particular, the concentrations of defects, their carrier-capture parameters, and energy levels can be found from PL measurements. The Kelvin probe method allows us to accurately determine the conductivity type for GaN thin films where Hall effect measurements are ambiguous. Temperature-dependent surface photovoltage (SPV) studies provide useful data about the surface band bending in dark, during illumination and the subsequent restoration after illumination. Using these methods, we have studied Mg-doped p-type GaN samples grown by various techniques. Interestingly, the PL spectra at low temperatures for several different samples contain different dominant bands – the ultraviolet luminescence (UVL), blue luminescence (BL) and green luminescence (GL) bands – which are all related to different defects. In some samples, the UVL band is quenched abruptly at a characteristic temperature (between 100-200 K) which can be tuned by excitation intensity. In other samples, the UVL band demonstrates no tunable quenching and a much faster decay (orders of magnitude) after pulsed excitation in time-resolved PL experiments. Our temperature-dependent Kelvin probe measurements show a conversion of the conductivity type at low temperatures under UV illumination. The temperature of the change in conductivity type varies between the samples and is also tunable with excitation intensity. This is consistent with PL results and can be explained by a switch in the majority charge carrier. These results are indicative of some special features of the nonradiative defects in p-type GaN (deep donors which have large capture cross-sections for both carriers). Although all the samples are Mg-doped and have comparable p-type conductivity, we assume that unidentified point defects in Mg-doped GaN are responsible for the diverse behaviors which have been observed. By modeling and comparing data from PL and Kelvin probe studies, we can acquire a better understanding of the properties of p-type GaN materials.